What are redox reactions?

Redox reactions are chemical reactions in which oxidation and reduction occur simultaneously.

Give two examples of real-life redox reactions.

1) Rusting of iron: 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃

2) Combustion of fuels: CH₄ + 2O₂ → CO₂ + 2H₂O

What is oxidation?

Oxidation is the addition of oxygen, removal of hydrogen, or loss of electrons.

What is reduction?

Reduction is the removal of oxygen, addition of hydrogen, or gain of electrons.

Give an example where oxidation involves the addition of oxygen.

2Mg + O₂ → 2MgO
Magnesium is oxidized as it gains oxygen.

Give an example where oxidation involves the removal of hydrogen.

2H₂S + O₂ → 2S + 2H₂O
Here, hydrogen is removed from H₂S, so it is oxidized.

How is oxidation defined in terms of electron transfer?

Oxidation is the loss of electrons. Example: Na → Na⁺ + e⁻

How is reduction defined in terms of electron transfer?

Reduction is the gain of electrons. Example: Cl₂ + 2e⁻ → 2Cl⁻

What is an oxidizing agent?

A substance that gains electrons and gets reduced. Example: Oxygen, chlorine, nitric acid.

What is a reducing agent?

A substance that loses electrons and gets oxidized. Example: Hydrogen, carbon, sodium.

What is oxidation number?

The oxidation number is the charge an atom would have if all bonds were purely ionic.

What are the rules for assigning oxidation numbers?

• Free elements have oxidation number zero (e.g., O₂, H₂, Fe).
• Monoatomic ions have oxidation numbers equal to their charge (e.g., Na⁺ = +1, Cl⁻ = -1).
• Oxygen is -2, except in peroxides (-1) and with fluorine (+2).
• Hydrogen is +1, except in metal hydrides (-1).
• The sum of oxidation numbers in a neutral compound is zero.

Find the oxidation number of Fe in Fe₂O₃.

Oxygen = -2, so:

2x + 3(-2) = 0

2x - 6 = 0

x = +3

Thus, Fe has an oxidation number of +3.

What are the different types of redox reactions?

Combination reactions – Two substances combine to form one.

C + O₂ → CO₂

Decomposition reactions – A single substance breaks into two or more.

2H₂O₂ → 2H₂O + O₂

Displacement reactions – One element replaces another in a compound.

Zn + CuSO₄ → ZnSO₄ + Cu

Disproportionation reactions – One element is oxidized and reduced in the same reaction.

2H₂O₂ → 2H₂O + O₂

What are the two methods to balance redox reactions?

Oxidation Number Method – Change in oxidation numbers is balanced.

Half-Reaction Method – Oxidation and reduction half-reactions are balanced separately.

Balance the reaction using the oxidation number method:
Cr₂O₇²⁻ + Fe²⁺ + H⁺ → Cr³⁺ + Fe³⁺ + H₂O

Identify oxidation states:

Cr₂O₇²⁻ (Cr = +6) → Cr³⁺ (Cr = +3)

Fe²⁺ → Fe³⁺

Oxidation number changes:

Cr⁶⁺ decreases by 3 per atom (total 6 for 2 Cr).

Fe²⁺ increases by 1 per atom.

Balance charge using electrons and H⁺ ions:

Cr₂O₇²⁻ + 6Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O

What are some common redox reactions in daily life?

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Corrosion of metals: Fe + O₂ + H₂O → Fe₂O₃ ⋅ xH₂O

Photosynthesis: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂

What is a redox titration?

A redox titration determines the amount of an oxidizing or reducing agent in a solution using a redox indicator.

Give an example of a redox titration.

Permanganate titration: MnO₄⁻ + Fe²⁺ + H⁺ → Mn²⁺ + Fe³⁺ + H₂O

What is standard electrode potential (E⁰)?

It is the voltage associated with a reduction reaction at standard conditions (1M, 1 atm, 25°C).

What does a positive E⁰ value indicate?

A higher tendency to gain electrons (stronger oxidizing agent).

Arrange the following in order of increasing oxidizing power: Zn²⁺, Cu²⁺, Ag⁺.

Oxidizing power depends on the standard reduction potential (E°)—a higher E° means a stronger oxidizing agent.

• Zn²⁺ (E° = -0.76V) is the weakest oxidizing agent.
• Cu²⁺ (E° = +0.34V) is stronger than Zn²⁺.
• Ag⁺ (E° = +0.80V) is the strongest oxidizing agent.

Since oxidizing power increases with E°, the order is: